[zeroping]

The only reason to heat a water tank rather than on demand is if your electrical connection can't supply enough power for peak loads. This can be true for domestic hot water, like showers and sinks. Tankless hot water heaters exist (and are cheap), but take 3-4x as much as much instantaneous power, and can be expensive to install for that reason.

If this microwave tech has a COP of less than 1 (meaning it doesn't generate more thermal heating than you put in), then it doesn't solve this problem and better than resistive electric heating. You have the same electrical installation problems.

There may be some other advantage here (long-term maintenance?), but not power efficiency. Thermodynamics says so.

The only way to do better is to make something else colder while you heat your house, using some kind of heat pump.